How do you control AWS costs?

This is asked in almost every AWS interview because runaway spend is so common.

Visibility first — you cannot manage what you cannot attribute:

• Tagging enforced by policy for environment, owner, and cost centre, with cost allocation tags activated.
• AWS Budgets with alerts, and Cost Explorer for trend analysis. Anomaly detection catches a spike within a day rather than at month end.

Then the levers, in rough order of payoff:

• Turn off what is idle. Non-production environments running nights and weekends, unattached EBS volumes, old snapshots, idle load balancers, and unassociated Elastic IPs. This is usually the fastest large saving.
• Rightsize. Compute Optimizer gives specific recommendations; instances are routinely provisioned for a peak that never occurs.
• Savings Plans or Reserved Instances for predictable baseline load, and Spot for fault-tolerant batch work, which can cut compute cost dramatically.
• S3 lifecycle policies and Intelligent-Tiering to move cold data down the storage classes.
• Watch data transfer. Cross-AZ and egress charges surprise people because ingress is free, and a chatty architecture across availability zones can cost more than the compute.

Note: Framing cost as an architectural property — that an efficient design is a cheaper design — is stronger than treating it as a procurement exercise.